Key takeaways
- A ground mount is a free-standing structure: the plan set carries a foundation design, a racking plan and a trench detail no rooftop set needs.
- Foundation type drives the drawings. Driven pier, ground screw, concrete pier and ballast each demand a different schedule and capacity basis.
- Wind is usually worse on open ground. A field site is commonly Exposure C under ASCE 7-16 Section 26.7.3 where the same customer’s roof would have been Exposure B.
- NEC 2023 Table 300.5(A) governs the run back to the point of interconnection — in the general case, 18 in. of cover for nonmetallic raceway, 6 in. for RMC or IMC, 24 in. for direct-buried conductors.
- Ground mounts frequently trigger zoning review on a separate calendar from the building permit, and ground-mount structural work is one of the most common PE-stamp triggers.
Ground-mount solar plan sets differ from rooftop sets almost entirely below the modules. A rooftop array borrows an existing structure, an existing load path and an existing permit history. A ground mount brings its own: a foundation, a free-standing structure, a buried circuit back to the service, and a footprint the zoning code has an opinion about. The sheets above grade look familiar. The sheets below grade are new.
What changes when a solar array leaves the roof?
The structural and civil scope, not the electrical scope. Our ground mount plan sets carry a cover sheet, site plan, racking plan, electrical plan, assembly details, single line diagram and labels, with engineering stamps as an add-on. Set that against what a rooftop set contains and the substitutions are clean: the roof plan becomes a racking plan, the attachment detail becomes a foundation detail, and a trench section appears with no rooftop equivalent.
The electrical work barely moves. Conductor sizing, overcurrent protection and interconnection follow the same rules whether the modules sit on asphalt shingle or driven steel, and which NEC articles govern a PV plan set does not change with mounting method.
What changes is sequencing. On residential solar plan sets for a roof, everything follows from a site survey and framing dimensions. On a ground mount, the foundation sheet waits until somebody knows what the soil does.
Which foundation type does the plan set have to detail?
Whichever one the soil and the racking vendor agree on — and the drawings have to show its capacity basis, not just its geometry. Four types cover almost every small commercial and residential array.
| Foundation | How it carries load | What the sheets have to show |
|---|---|---|
| Driven pier | Skin friction and lateral soil resistance on the embedded length | Pile schedule with embedment and reveal per location, refusal criteria, galvanizing spec, lateral capacity basis, load-test requirement |
| Ground screw / helical anchor | Helix bearing plus shaft friction | Screw model and length, installation torque criteria, embedment, test frequency, minimum edge distance |
| Cast-in-place concrete pier | End bearing plus skin friction | Pier diameter and depth, concrete strength, reinforcement schedule, anchor-bolt detail, uplift and overturning capacity |
| Ballasted on grade | Dead weight resisting uplift, sliding and overturning | Ballast weight and block count per table, layout by wind zone, allowable bearing pressure on subgrade, grade prep and drainage notes |
The choice is not a preference. Driven piers are fast where soil takes them and useless in rock or obstructed fill. Ground screws tolerate shallow rock. Concrete piers work almost anywhere and cost the most in cure time. Ballast is the fallback where you cannot penetrate at all.
What a reviewer checks is that the schedule ties back to something. An embedment number with no soil parameter behind it is a number somebody made up. Same discipline as any other design factor to settle first: state the assumption, and give it a source.
When does a geotechnical report become the gating item?
As soon as the foundation is a deep foundation, which driven piers and ground screws almost always are. Under IBC 2021 Section 1803.5.5, reproduced in the City of Seattle’s 2021 Building Code Chapter 18, “where deep foundations will be used, a geotechnical investigation shall be conducted” unless sufficient data on which to base the design and installation is otherwise available.
What that investigation produces reads like a foundation schedule: recommended foundation types and installed capacities, centre-to-centre spacing, driving criteria, installation procedures, field inspection and reporting, load test requirements, material suitability, the bearing stratum, and reductions for group action. Most of those nine items land on a drawing.
The escape hatch matters as much as the rule. That qualifier is what lets a vendor’s soil-class table stand in on a small array in known ground, and IBC 2021 Section 1803.2 lets the building official waive the investigation where adjacent-site data makes it unnecessary. Which way your reviewer leans is worth asking early, like any other permitting nuance by jurisdiction.
Before anyone mobilises a rig, the USDA Natural Resources Conservation Service publishes free soil mapping through Web Soil Survey, covering most U.S. counties. No engineer will accept it as a geotechnical report, but it will tell you which jobs need one before you quote them.
How does frost depth change the embedment?
Frost sets a floor under anything bearing near the surface, and it forces a detail on the conduit riser. IBC 2021 Section 1809.5 gives three ways to protect a foundation from frost: extend it below the frost line of the locality, build in accordance with ASCE 32, or bear on solid rock. It also bars a shallow foundation from bearing on frozen soil unless that condition is permanent.
Note “of the locality.” There is no national frost depth. The AHJ or a state amendment publishes it, from nothing on the Gulf coast to past 40 in. across the northern tier. Put it on the cover sheet with its source.
For driven piers and ground screws, frost usually governs nothing — embedment is already several feet down because lateral capacity demanded it. The trap is everything else: the equipment pad, a shallow ballast footing, a bollard protecting the inverter. Those heave unevenly across a long row, which is how a rack that was flat in October is out of plane by March.
The conduit has the same exposure. NEC 2023 300.5(J) requires raceways subject to settlement or frost heave to be arranged so conductors and equipment are not damaged. New York’s NY-Sun programme puts it plainly in its 2023 NEC field inspection reference: conduit below grade is installed with provisions for movement, such as a frost sleeve. That is a drawn detail, not a field decision.
Why is wind loading harsher on open ground?
Because exposure category usually goes up, and exposure moves the design pressure with it. ASCE 7-16 Section 26.7.3 defines Exposure B as urban and suburban terrain with numerous closely spaced obstructions the size of single-family dwellings or larger, and Exposure C as open terrain with scattered obstructions generally under 30 ft tall. A retrofit on the customer’s roof in a subdivision is often B. The ground mount in their back pasture is C. Same address, same wind map, higher pressures.
Which ASCE 7 edition applies is pinned to the code cycle the AHJ adopted, not the calendar: the 2021 IBC references ASCE 7-16, the 2024 IBC references ASCE 7-22. Name the edition on the sheet; the two do not produce the same numbers.
Ground mounts also sit in a gap in the standard. ASCE 7-16 has no provision for free-standing arrays the way Sections 29.4.3 and 29.4.4 cover rooftop panels, so engineers fall back on Chapter 29’s freestanding-wall and sign provisions, or a wind tunnel study. ASCE 7-22 adds qualifying criteria for fixed-tilt ground-mounted systems; single-axis trackers stay outside them.
Snow behaves differently too. No roof to shed onto and no building heat below, so the array carries closer to full ground snow load, and drift can build against the low edge until it loads modules in bending — which is why low-edge ground clearance is a dimension on the racking plan. Do not mix editions here either: ASCE 7-22 reissued the ground snow maps on a different design basis. The underlying methodology is in our piece on structural load calculations.
Where does the racking manufacturer’s engineering stop?
At grade, and inside a stated envelope. A manufacturer’s certification letter and span tables cover the rack: member sizing, allowable spans, module clamping, the bonding path, and the wind and snow range the system was evaluated against. That is real engineering and worth having. It also stops short of your site.
IronRidge says so in its own ground mount installation manual, version 4.9. Installers are told to validate foundation parameters before installation, “as a local geotechnical report may be required to assess ground conditions,” and to consider “consulting with a local engineer familiar with local regulations and build site requirements, including soil conditions, terrain and load criteria.” The vendor engineered the rack. Nobody has engineered the connection between that rack and this field.
Three things routinely fall outside the certification: the foundation — embedment, capacity, spacing and the soil parameters behind them; out-of-envelope conditions — wind speed, exposure, ground snow, tilt or clearance beyond what was evaluated; and as-built geometry — odd row spacing, sloped ground, mixed foundation types in one array.
The honest part: ground-mount structural work is one of the most consistent PE-stamp triggers there is. The triggers are covered in when a PE stamp applies; stamped review runs through our engineering stamps service with a PE licensed in the project’s state.
What does the trench and conduit run have to show?
Cover depth by wiring method, protection where conductors emerge, provision for earth movement, and working space at whatever the trench lands on. That run is too often drawn as a dashed line with no dimensions.
Cover depths come from NEC 2023 Table 300.5(A), in NFPA’s NFPA 70, the National Electrical Code. For locations not otherwise specified: 24 in. for direct-buried cables or conductors, 6 in. for rigid metal or intermediate metal conduit, and 18 in. for nonmetallic raceways listed for direct burial without concrete encasement. The 2023 cycle added electrical metallic tubing to that third column. Under streets, roads, driveways and parking lots the table requires 24 in. for every method; a driveway serving only a one- or two-family dwelling gets its own row at 18 in. Settle which row your gravel lane falls into before the trench is drawn.
Three more requirements belong on the same detail. NEC 2023 300.5(D)(4): conductors emerging from grade need an enclosure or raceway from the minimum cover depth to at least 8 ft above finished grade. NEC 2023 300.5(B): an underground raceway is a wet location, so conductors must be listed for wet use. NEC 2023 300.5(J): provision for settlement and frost heave. Voltage drop is uncodified — NEC 2023 90.5(C) makes informational notes non-enforceable — but reviewers still look for the calculation.
Inverters, disconnects or a combiner on a pad at the array need NEC 2023 110.26 working space dimensioned: 3 ft of depth at 150 V or less to ground under Condition 1, 30 in. of width or the equipment width, 6.5 ft of headroom. Array-to-structure separation and distance to property lines come from zoning and the local fire code, and both belong on the site plan — leaving them off is one of the avoidable plan set mistakes that generate a round trip for nothing.
Do ground mounts trigger zoning and land-use review?
Frequently, yes — on a separate calendar from the building permit. A rooftop array is almost always an alteration to an existing building. A ground mount is a new structure on the lot, which puts it in front of the planning department too.
The recurring items:
- Accessory-structure setbacks — side, rear and front-yard minimums applied to the array footprint, sometimes measured to the module edge at full tilt rather than the foundation.
- Lot coverage — some jurisdictions count the whole array footprint, others only the foundations. On a small lot that difference is the project.
- Height limits — measured to the top of the array at maximum tilt.
- Screening and buffers — planting or fencing where the array faces a road or a neighbour.
- Conditional use — larger arrays in residential or agricultural zones sometimes need a discretionary approval with a hearing.
None of this is in the NEC or the IBC. It lives in the local zoning ordinance, and it is the most common reason a structurally straightforward ground mount misses its install date. Confirm the zoning path when you confirm the code cycle; on larger sites the same review sits on the critical path for utility-scale PV system design. For Texas work, where open-field racking is routine, our Texas solar plan sets page covers the state-specific side.
Worked example: a 19.2 kW ground mount 140 ft from the service
Forty-eight modules at 400 W — 19.2 kW DC — on a two-row fixed-tilt rack behind a rural house, feeding a 200 A service 140 ft away. Two 7.6 kW string inverters sit on a pad at the array.
Foundation. Twenty-four driven piers. The soils report calls for 7 ft of embedment for lateral capacity, and published frost depth is 42 in., so the piers are well below it. The equipment pad is the problem: at 12 in. thick it sits inside the frost zone, so under IBC 2021 Section 1809.5 it has to extend below 42 in. or be built to ASCE 32 — one of the most common omissions in a ground-mount set.
AC circuit. Combined inverter output is 15.2 kW, or 63.3 A at 240 V. NEC 2023 690.8(B) sizes the conductor and overcurrent device at 125% of continuous current: 63.3 x 1.25 = 79.2 A, so an 80 A device. Copper THWN-2 at the 75 degree C column gives 4 AWG an ampacity of 85 A. Over 140 ft one way it drops roughly 2.3%; 3 AWG about 1.8%.
Trench. The route runs 125 ft through pasture and crosses 15 ft of gravel drive. In the open section, PVC listed for direct burial needs 18 in. of cover per NEC 2023 Table 300.5(A). Under the drive it depends which row the AHJ applies: 18 in. as a dwelling driveway, 24 in. as a road or parking area. Draw it at 24 in. and the question never gets asked. At both risers, protection to 8 ft above grade per 300.5(D)(4) and a frost sleeve per 300.5(J).
What the set carries. Racking plan with the 24-pier layout and foundation schedule; assembly and foundation details; a trench section with cover, warning tape and riser protection; a site plan with separations dimensioned; the single line; labels. The foundation sheet cannot start until the soils data exists — the scheduling lesson of every ground mount.
FAQ
What’s different about a ground-mount solar plan set?
A ground mount is a free-standing structure, so the set carries a foundation design, racking plan, assembly details and a trench section that a rooftop package does not. The electrical scope is broadly the same.
Do ground mounts need a soils report?
Often. Under IBC 2021 Section 1803.5.5 a geotechnical investigation is required where deep foundations are used, which covers driven piers and ground screws, unless sufficient data is otherwise available. Some AHJs accept a vendor’s soil-class table on small arrays.
How deep does conduit to a ground-mount array have to be buried?
Per NEC 2023 Table 300.5(A), in the general case: 24 in. for direct-buried conductors, 6 in. for rigid metal or intermediate metal conduit, and 18 in. for nonmetallic raceway listed for direct burial. Under roads, driveways and parking lots the table requires 24 in., with a separate 18 in. row for dwelling driveways.
How does frost depth affect a ground-mount foundation?
IBC 2021 Section 1809.5 requires frost protection by extending below the local frost line, by construction in accordance with ASCE 32, or by bearing on solid rock. Driven piers and ground screws are usually below frost already; equipment pads and shallow ballast footings are what heave.
Does a ground mount need zoning approval as well as a building permit?
Frequently. A ground mount is a new structure on the lot rather than an alteration to an existing building, so it commonly triggers accessory-structure setbacks, lot coverage limits, height limits and sometimes a conditional use permit, on its own timeline.
Get the foundation sheet drawn before the rack ships
What delays a ground mount is rarely the array. It is the foundation sheet nobody could draw, because the soils data did not exist.
Avila Solar Drafting produces permit-ready solar plan sets in 2-3 business days, backed by our guarantee of accurate solar plan sets and six months of free revisions from the order date. Ground-mount structural work is scoped case by case, because the foundation is where the variability lives.
Send us the site data you have and we’ll tell you what’s missing before it becomes a correction.
Unusual soils, a stamped structural scope, or a multi-acre footprint? Call 971-410-0655 and we’ll scope it directly.
